This invention relates to a continuous rolling mill, in particular a rolling mill having rolling units with three rolls for accomplishing the initial rolling passes on an ingot originating from a continuous casting machine of the wheel and band type.
The geometry and shape of the cross-section of an ingot leaving a machine of this type is determined substantially by two factors. Firstly, it has an upper face which is necessarily flat in that it is formed in contact with the metal band which in these machines closes the groove in the casting wheel over a certain arc, and secondly it comprises two lateral faces formed in contact with the lateral walls of the casting groove, which of necessity form a draft angle with the normal to the upper flat face to enable the ingot to leave the casting wheel.
The draft angle, normally 8.degree., cannot be varied to any sensible extent, and the only freedom remaining to the designer who wishes to modify the cross-section of the ingot in any way in order to better adapt it to the rolling requirements to which the ingot will be subjected is to vary the height/width ratio of the cross-section and the more or less rounded shape of the part opposite the flat face, which shape reproduces the shape of the bottom of the casting groove.
In rolling these ingots in relation to the particular shape of their cross-section, various methods have been proposed for the initial rolling passes. The most known and widespread methods are those which use rolling units having three rolls disposed at 120.degree. to each other and defining a passage substantially in the shape of a flat sided hexagon or triangle, or a passage obtained by combining the two latter.
In the first case the rolls comprise a groove having a cross-section consisting of straight lengths disposed at approximately 120.degree. to each other and converging towards the middle, where they are joined together to form the groove base. The two lengths define substantially two sides of the hexagonal passage, the other sides being formed respectively by the contours of the grooves of the other two rolls. In the second case the rolls are absolutely free from any rolling groove and have a perfectly cylindrical geometrical shape, the lateral surfaces of the rolls defining the rolling surface. In the third case two rolls of the first type and one of the second type are used in practice.
All these methods, though to different extents, create in the recently molten ingot material slip towards the free space between one roll and another, this slip being sufficient to create cracks in the case of certain metals, such as copper, at the accumulation areas formed between the rolls. Furthermore, these methods are not able to eliminate the small central holes which may form or are formed in casting certain alloys, such as aluminium alloys, e.g. aluminium-magnesium alloys.